The invention relates to a mid frequency high power transformer, particularly for use as a welding transformer for resistance welding, with at least one primary winding through which one limb of a transformer core passes and a secondary winding, which windings are constructed as concentric cylindrical windings situated above one another.
Connecting together workpieces of metal plate is currently generally effected by resistance welding, welding robots preferably being used in mass production which carry welding tools for spot welding, e.g. of body work panels etc., at the free end of a working arm which may be moved into practically any desired working position. The welding current is provided by welding transformers with a voltage which is matched to the welding tongs, which are generally of low impedance, i.e. the welding transformers supply very high currents at relatively low secondary voltages at their output. The welding transformers should be disposed in the vicinity of the welding tongs, i.e. should be moved with the working arm of the robot. A prerequisite for this is as low as possible a weight of the welding transformers in order to prevent the robot arms having to be constructed very massively and thus heavily as a result of the weight loading and in order to ensure that the forces necessary for the precise movement of the working arms can be maintained as small as possible.
When resistance welding, the available mains alternating voltage, with a frequency of 50 or 60 Hz. is conventionally transformed to the necessary low welding voltage by means of single phase transformers. If high welding currents are necessary or light welding current sources are required, mains frequency transformers are unsuitable since relatively large transformer cores are necessary as a result of the low frequency. So-called mid frequency welding systems have thus been developed with operating frequencies of 300 Hz. to about 5 kHz. The higher frequencies permit a considerable reduction in size of the transformer cores in this case and, associated therewith, a considerable reduction in weight. Mid frequency transformers have previously been operated nearly exclusively with a rectifier connected to their outlet side in order to compensate for the inductive losses in the welding tongs which increase with frequency. Since such transformers are generally water cooled, a pancake coil construction is generally selected. The secondary windings are mostly constructed in the form of a hollow profile of rectangular cross-section, into the interior of which the cooling medium is conducted. On the other hand, the cooling of the primary winding is effected indirectly in that the pancake coils are wound in a plurality of layers from flat copper strip and are disposed in the spatial vicinity of the secondary coil. In so far as mid frequency welding transformers with cylindrical windings have been proposed in individual cases (DE 2549327 A1), attempts have been made to conduct away the heat losses which occur by causing a cooling medium to flow against the primary and secondary windings, conducted in cooling passages defined between them. As regards the magnetic coupling between the primary and secondary circuits and the achievement of as homogeneous current densities as possible in the windings, an optimal result is not achieved in this construction, as also when using pancake windings, the higher operating frequencies leading in particular to high losses in the transformer and thus to a reduction in efficiency.
Against this background, it is the object of the invention to provide a high power transformer with a low secondary voltage which may be used for resistance welding or resistance heating and may also be used as a welding transformer for welding robots operating with welding tongs by reason of its low weight, whereby the components necessary for rectification of the secondary current, which is desirable or necessary when resistance welding, are to be capable of being integrated into the transformer unit in a simple manner.
Starting from a transformer of the type referred to above, this object is solved in accordance with the invention if the turns of the primary coil(s) are constituted by a plurality of layers of an electrically conductive metallic strip material of relatively small thickness spirally wound on top of one another and the turns of the secondary winding are constituted by a smaller number, selected in accordance with the desired step down ratio of the primary to secondary voltage, of layers of an electrically conductive, metallic strip material of greater thickness wound on top of one another, the breadth of the winding layers of the strip materials, measured at right angles to the longitudinal direction, is substantially equal to or only slightly smaller than the length of the limb of the transformer core passing through the winding, and if the connecting lines on the secondary side are connected to the end face edges of the turns of the strip material of the secondary coil. The primary winding is thusxe2x80x94like the secondary windingxe2x80x94constituted by spiral layers of strip material wound on top of one another, e.g. strip-shaped bands of copper plate, the primary winding being constituted by a plurality of layers of thin strip material and the secondary winding being constituted by a smaller number of layers of thicker strip material with a relatively large cross-sectional area selected in accordance with the desired step-down ratio of primary to secondary voltage in order to conduct the high secondary currents away at low voltage with as low a resistance as possible.
In a preferred embodiment of the invention, the transformer is constructed as a single phase transformer of shell-type including a transformer core with three parallel limbs, which are spaced from one another and are connected at both ends by a respective web, whereby conveniently provided on the central limb of the transformer core there is firstly a first inner primary winding, radially adjoining a second winding, which concentrically surrounds the radially inner primary winding, and then a concentric, outer, secondary primary winding, which radially adjoins the secondary winding.
The connections of the transformer on the secondary side can advantageously be constructed in the form of strips of electrically conductive, metallic strip material, which are integrally attached to the edges of the winding layers of the secondary winding and which are bent outwardly approximately at right angles in a radius from the respective associated edge and at right angles to the central plane defined by the three limbs of the shell core over the respective associated end surface of the transformer winding. The shape of these connecting elements is thus disposed substantially in the direction of the field lines extending outside the core.
The connections of the transformer on the secondary side can have two respective strips of conductive, metallic strip material, which are attached to opposing edges of the winding layers of the secondary winding and are bent outwardly over the respective associated end surface of the transformer windings approximately at right angles from the respective associated edge and at right angles in a radius to the central plane defined by the three limbs of the transformer core, whereby the free ends remote from the windings of each pair of connecting strips connected to the same winding layer are then conveniently connected together by a respective common connecting plate of electrically conducted material, which extends over the winding substantially parallel to the central limb of the transformer core.
In a preferred embodiment of the invention the secondary windings have a total of three connectors or connecting plates which are connected to the radially innermost, the radially outermost and a central winding layer (centre tapping) of the secondary winding.
The connecting plates connecting the connecting strips connected to the radially innermost winding layer of the secondary winding and those connected to the radially outer most winding layer of the secondary winding are then conveniently arranged in a plane laterally offset from one another whilst the connecting plate connecting the connecting strips connected to the central winding layer is arranged in a parallel plane offset with respect thereto.
It is thus possible to integrate into the transfomer the rectifier circuit necessary for the rectification of the secondary current if connected to the flat surfaces remote from the coil of the connecting plate associated with the radially inner most layer of the secondary winding and with the radially outer most winding layer of a secondary winding there is a connector contact of at least one respective semi-conductor rectifier element, whose other electrical connections extend together to a common electrical connector of the secondary winding.
If the rectifier elements are constructed in the form of disk shaped high power semi conductor diodes with a respective connector contact surface provided on the opposed flat surfaces, then the construction can be such that the semi conductor diodes are in electrical contact with a connector contact surface on the surface remote from the windings of one of the parallel connecting plates of the secondary winding whilst their respective other connector contact surfaces are in electrical contact with the flat surface, directed towards them, of the parallel, offset contact plate, constituting the common connector.
In order to construct the contacts with as low a resistance as possible, the semi conductor diodes are conveniently clamped between the electrical connecting plates and the common connector contact plate, pre-stressed spring elements conveniently being used for this purpose.
In order to conduct away the heat losses, it is recommended that the transformer be enclosed in a housing, the interior of which is constructed substantially complementary to the outer shape of the transformer and whose walls are provided with passages for the flow of a liquid or gaseous cooling medium.